Tires for passenger vehicles generally comprise two beads which are configured to come into contact with a mounting rim, as well as two sidewalls extending the beads radially to the outside and being joined together in a crown. A carcass reinforcement comprising a plurality of elements for reinforcing the carcass is anchored in the two beads and extends across the sidewalls towards the crown. The crown comprises a crown reinforcement, generally formed by at least two reinforcing layers each comprising a plurality of thread-like crown reinforcing elements, said crown reinforcing elements being parallel to one another in each layer and the reinforcing elements of one layer extending crosswise with respect to those of the other. The crown reinforcement is surmounted by a tread designed to come into contact with the ground when the tire rolls along.
It is well known that in harsh conditions of use, gradually as the distances covered increase, fissures may appear at the ends of the thread-like elements forming the reinforcing layers of the crown reinforcement. Said fissures may then spread along the thread-like elements, then join up between two or more thread-like elements and thus result in a separation of the lateral ends of the two crown reinforcing layers.
In order to limit the initiation and spread of these fissures, strips of rubber mix are usually inserted between the ends of the reinforcing layers, the rubber mix of the strips having identical modulus of elasticity, or even lower modulus of elasticity than that of the layers of rubber mix of the reinforcing layers of the crown reinforcement.
The document U.S. Pat. No. 6,640,861 reveals a tire the crown structure of which is capable of limiting the phenomena of separation of the lateral ends of the crown reinforcing plies such that they occur only at significant levels of stress. This object is achieved by a particular arrangement of the rubber mixes provided between the thread-like elements of the crown reinforcement. The threads of at least one of the two reinforcing layers are successively in contact, on the same side as the layer, moving axially from the median plane of the tire towards the lateral ends of the reinforcing layer with, at least, a first layer of rubber mix having a first modulus of elasticity (typically between 9 and 13 MPa), then a second layer of rubber mix having a second modulus of elasticity lower than the first (typically less than 5 MPa). This second layer has the advantage of better resisting the formation of fissures in the lateral end zones of the reinforcing layers which are considerably stressed.
A similar structure is described in the document U.S. Pat. No. 6,776,205 which discloses a tire having reduced rolling resistance. Between the superimposed reinforcing layers of the crown reinforcement are located, arranged axially in an adjacent manner, at least two layers of rubber mix having different mechanical properties, each of the layers being in contact with the threads of the superimposed reinforcing layers. According to a preferred embodiment disclosed, the ratio between the moduli of elasticity of the second layer and of the first layer (of which the modulus is between 10 and 15 MPa) is between 0.05 and 0.8, and even more preferably between 0.5 and 0.7.
A drawback of these solutions is in the fact that these tires have the tendency to exhibit irregular wear, associated with the fact that the flattening of the tire is insufficient, which leads to premature wear in the centre of the tire. To reduce this drawback, it is known to provide a small hooping reinforcement (also known as a bracing layer) generally made of polyester, in the central part of the crown. The presence of this hooping layer has, however, the drawback of complicating the tire manufacturing process and increasing its cost.